If it's a play - and from the way the question is asked I suspect it will be - then generally a royalty has to be paid to the author or owner of the copyright unless the copyright has expired, which is usually 70 years after the death of the author. However, levels of royalty vary depending on size of audience, length of play etc., and some authors may be willing to let amateurs perform with no royalty, although this does not usually apply with popular pieces that are also put on by professionals. The best people to advise about this are the publishers of the any work that is under consideration. I was once in a play of a Terry Pratchett Discworld novel which we were allowed to perform without payment, but I believe that a royalty may now be payable for these plays. It's worth checking, of course. What Sexy Jag is saying may apply if you use a venue that organises copyright payment if you hire it, but I suspect that your charity may in that case be expected to pay a fair whack for hiring the venue.
www.samuelfrench-london.co uk - forgot the www last time, sorry.